1. Field of the Invention
The invention relates generally to well logging. More specifically, the present invention relates to methods and systems for interpreting data obtained with well logging tools.
2. Background Art
Determining the porosity and fluid content of subsurface earth formations are critical elements in maximizing the efficiency of oil, gas, and water (xe2x80x9cformation fluidsxe2x80x9d) exploration. In order to improve formation fluids exploration, drilling, and production operations, it is necessary to gather as much information as possible on the properties of the underground earth formations as well as the environment in which drilling takes place.
The collection of downhole information, also referred to as logging, is realized in different ways. A well tool, comprising sources and sensors for measuring various parameters, can be lowered into the borehole on the end of a cable, or wireline. The cable, which is attached to a mobile processing center at the surface, is the means by which parameter data is sent up to the surface. With this type of xe2x80x9cwirelinexe2x80x9d logging, it becomes possible to measure borehole and formation parameters as a function of depth, i.e., while the tool is conveyed along the wellbore.
An improvement over wireline logging techniques is the collection of data on downhole conditions during the drilling process. By collecting and processing such information during the drilling process,the driller can modify or correct key steps of the operation to optimize performance and avoid financial injury due to well damage such as collapse or fluid loss. Formation information collected during drilling also tends to be less affected by the drilling fluid (xe2x80x9cdrilling mudxe2x80x9d) invasion processes or other undesirable influences as a result of borehole penetration, and therefore are closer to the properties of the virgin formation.
Schemes for collecting data of downhole conditions and movement of the drilling assembly during the drilling operation are known as measurement-while-drilling (MWD) techniques. Similar techniques focusing more on measurement of formation parameters than on movement of the drilling assembly are known as logging-while-drilling (LWD). However, the terms MWD and LWD are often used interchangeably, and use of either term herein includes both the collection of formation and borehole information, as well as data on movement of the drilling assembly. Other logging operations are carried out using coiled tubing, slick lines, logging-while-tripping, and permanent monitoring applications as known in the art.
Conventional well-logging tools, such as the resistivity tools disclosed in U.S. Pat. No. 4,899,112, issued to Clark et al., provide an extensive set of measurements that contain valuable information on the underground formations and on environmental parameters. The data must be interpreted to extract the useful information. One of the more powerful techniques for interpreting well log data is xe2x80x9cinversionxe2x80x9d processing. Inversion of well log data is described in a number of patents. See, for example, U.S. Pat. Nos. 6,047,240, 5,345,179, 5,214,613, 5,210,691, and 5,703,773.
Inversion processing techniques known in the art typically involve determining the spatial distribution of physical properties of earth formations surrounding the well logging instrument. Inversion processing, as shown in FIG. 1, generally includes making an initial model of the spatial distribution of formation properties (shown as 21), calculating an expected response of the well logging instrument to the model (shown as 22), and comparing (i.e., computing a difference) the expected response to the measured response of the logging instrument (shown as 23). If the difference between the expected response and the measured response exceed a predetermined threshold (shown as 24), the model is adjusted and the process is repeated (shown as 26) until the differences fall below the threshold. The model, after adjustment that results in the reduced differences, then represents a likely distribution of properties of the earth formations (shown as 25).
Raw log data can be affected to varying degrees by any of several factors such as borehole effect, tool eccentering, shoulder-bed effects, invasion, anisotropy, etc. Identifying these effects and correcting for them is challenging, especially when conclusions are needed quickly to make operational decisions.
Methods such as that shown in FIG. 1 deal with a single model, which may contain only a limited number of environmental effects. In most cases it is not practical to include all possible environmental effects because the model would become too complex, leading to slow processing and/or problematic convergence. Using such methods, an operator first needs to identify the dominant environmental effect(s), based on the log patterns, before he can choose the right processing to apply. The success of this operation depends on the knowledge and experience of the operator and is time consuming. Often, differences in responses caused by different environmental effects are subtle and difficult to identify even by a specialist. Therefore, it is desirable to have techniques that can perform such evaluations without prior knowledge of which environmental factors have influence on the log data.
In one aspect, embodiments of the invention relate to methods for diagnosing environmental influence on well log data. The invention provides a method for diagnosing environmental effects in well log data includes selecting a plurality of models, each of the plurality of models including at least one parameter to be optimized; fitting the each of the plurality of models to the well log data by optimizing the at least one parameter, the fitting producing a plurality of optimized models; determining an indicator of goodness of fit for each of the plurality of optimized models; and selecting a model representing a best fit of the well log data based on the indicator.
Another aspect of the invention relates to systems for diagnosing environmental influence on well log data. The invention provides a system for identifying environmental effects in well log data includes a computer having a memory for storing a program, wherein the program comprising instructions executable by the computer for selecting a plurality of models, each of the plurality of models includes at least one parameter to be optimized; fitting the each of the plurality of models to the well log data by optimizing the at least one parameter, the fitting produces a plurality of optimized models; determining an indicator of goodness of fit for each of the plurality of optimized models; and selecting a model representing a best fit of the well log data based on the indicator.
Other aspects of the invention will become apparent from the following description, the drawings, and the claims.